Surface roughening is one of the efficient ways to improve light extraction efficiency of a light-emitting device. Roughening the substrate or the upmost semiconductor layer into irregular protrusions/depressions to scatter incident light impinging on the roughened surface is an example to improve light extraction efficiency. Roughening surface can be achieved by known processes like mechanically polishing or reactive-ion-etching (RIE). Another feasible way is performed by wet etching the wafer immersed in an etching solution for certain duration. The surface is roughened by different etching rates of the etching solution versus different exposed crystal planes of the surface. The roughened surface of the light-emitting device, as shown in FIG. 1, is formed by wet etching. The light-emitting device comprises a growth substrate 11, an n-type semiconductor layer 12, an active layer 13, a p-type semiconductor 14, a p-side conductive pad, and an n-side conductive pad. The surface of the p-type semiconductor layer 14 is wet-etched to form a roughened surface. An undercut 17 is formed due to lateral etching in the border between the p-side conductive pad 16 and the p-type semiconductor layer 14. The contact area between the p-side conductive pad 16 and the p-type semiconductor layer 14 is therefore reduced such that the device reliability is easily failed or the p-side conductive pad 16 is easily peeled off by the stress. Besides, the uniformity of the roughened surface formed by the conventional wet etching method is not good enough to keep the product stable.
Another conventional way to prevent the light-emitting device from reliability failure or pad peeling is to form the roughened surface before forming the conductive pad, but the contact resistance between the conductive pad and the roughened surface of the underlying layer becomes high and therefore downgrade the device performance. Besides, the resulted surface of the conductive pad is uneven and therefore obstructs the wire-bonding yield.